The present invention relates generally to integrated circuit technology, and more particularly to temperature-compensated resistors and fabrication methods therefor.
Gallium arsenide (“GaAs”) metal semiconductor field effect transistors (“MESFETs”) are widely used for amplification at microwave frequencies, high-speed digital switching, and various other demanding applications. The increasing use of microwave frequencies in satellite and wireless communications has accelerated the need for high-performance GaAs transistors and associated solid state, integrated circuit (“IC”) configurations. As the power output capability of MESFETs continues to improve, a single transistor can provide the power once provided by several transistors, thereby saving considerable costs and drastically reducing the size of the amplifier modules. The higher the power handling capability and efficiency that can be achieved, the greater will be the number of potential applications for MESFET amplifiers. Accordingly, there has been a tremendous effort in commercial and military industries over recent years to improve the performance of GaAs devices.
A conventional MESFET employs a metal gate electrode in direct contact with a GaAs substrate to form what is known as a Schottky barrier. A voltage applied to the gate electrode influences a current-carrying region beneath the gate. This controls the flow of current between the source and drain electrodes of the transistor, providing amplification or switching.
Field effect transistors (“FETs”), and more specifically high electron mobility transistor (“HEMT”) ICs have widespread application in many areas, including aerospace and mobile communication systems. When FET or HEMT ICs are exposed to variations in temperature, their electrical performance characteristics can vary. Many applications for such circuits require consistent operation over a broad range of temperatures.
To achieve temperature-invariant operation of FET or HEMT ICs, therefore, numerous temperature-compensation (“TC”) solutions have evolved, with varying degrees of success. Such solutions can generally be classified into three approaches.
The first approach involves the use of external temperature sensors and amplifiers to generate a control signal that is delivered to the IC. This is perhaps the most obvious TC method, and one that has been in use for decades. One of the main disadvantages of this method is that external circuits must be added for the TC function. These external circuits require temperature-variable elements, such as thermistors or other thermal sensors, that are not easily integrated on-chip. Additionally, the thermal sensors must be located in close proximity to the circuit that is being compensated in order to accurately provide a meaningful temperature reading. This proximity requirement further complicates the assembly of a temperature-compensated circuit embodying this solution.
The second approach utilizes an on-chip temperature sensor, such as a diode, in conjunction with an external feedback control loop. This is similar to the first approach except that a temperature-sensing element is integrated directly into the IC chip. The temperature sensor is typically a monolithic diode that is biased by a constant current from an external source to produce a terminal voltage inversely proportional to temperature. This voltage is amplified and used either to control the bias of active devices on the IC or to control a variable attenuator to adjust the response of the circuit as it changes with ambient temperature. The disadvantages of this method include the size, cost, and assembly complications arising from the addition of external control circuits, which typically require individual adjustment to match the temperature characteristics of each IC.
The third approach utilizes an on-chip direct-current (“DC”) feedback amplifier to regulate the bias of the IC circuits. In this approach, a DC amplifier is integrated directly into the IC. While this monolithic solution is the most compact of the three approaches, it has disadvantages with some IC applications, such as high-performance HEMT ICs with low DC power dissipation (used, for example, in satellite communications circuitry). Integrating the active bias regulator on-chip involves designing precise DC operational amplifiers into a semiconductor process that is optimized for high-frequency RF devices, not DC devices. Such high-frequency RF devices are not well suited for precise DC operational amplifiers. The resulting compromise produces inefficient bias regulation, which may more than double the power consumption of the IC.
A better approach would be to provide circuits that do not need separate TC. Such circuits, for example, would be inherently stable over the operating temperature ranges of interest. Unfortunately, this requires the ability to provide individual circuit components that can be tailored for specific temperature dependencies and response characteristics. Further, such circuit components should preferably be essentially interchangeable dimensionally and functionally with existing design configurations and component specifications. Unfortunately, components that meet these needs in affordable and readily manufacturable configurations have not been available in an adequately broad and commonly usable form.
Thus, a need still remains for temperature-compensating IC components, and in particular, temperature-compensating components such as IC resistors that are adapted for use in GaAs process ICs and IC designs. A particular need remains for methods and structures to be used for profiling the temperature response and the temperature compensation of such IC resistors in a controllable and reliable manner. In view of the ever-increasing need to save costs and improve efficiencies, it is more and more critical that answers be found to these problems.
Solutions to these problems have been long sought but prior developments have not taught or suggested any solutions and, thus, solutions to these problems have long eluded those skilled in the art.